Intracenter Transitions Research Articles

Morphology and Luminescence Properties of Transition Metal Doped Zinc Selenide Crystals.

Zinc selenide is an excellent matrix material to dope with rare-earth and transition metal to achieve mid-infrared luminescence to develop high power lasers. The luminescence, morphology and refractive index is significantly affected by the doping and defects generated due to size and valency of dopants, concentration, growth process and convection during the growth. The aim of the study is to investigate effect of point and line defects generated due to low doping of iron and chromium on the emission and morphology of the zinc selenide. Luminescence and morphological properties of large iron and chromium doped zinc selenide single crystals were studied to evaluate the effect of extremely low residual impurities and defects associated with the doping process. The emission properties following both short wavelength (i.e., ultraviolet; 350-370nm) excitation and longer wavelength (i.e., near infrared; 850-870nm) excitation were characterized. Luminescence emission bands were identified in both doped crystals. In addition to the primary emission bands, satellite peaks and intra-center transitions were also observed. Due to local population defects associated with the residual impurities (ppm to ppb) in the Fe-ZnSe and Cr-ZnSe crystals, peak emission wavelengths were observed to shift. The emission bands were found to decrease in intensity due to recombination of residual impurity co-dopants and complex defects generated during growth and fabrication. Cryogenic temperature analyses revealed a very clean emission band due to freezing of some of the point and line defects. An emission band observed at 980nm for both crystals at room temperature as well as cryogenic temperatures indicates a vibronic peak in ZnSe. The scanning electron microscopy (SEM) images of the local morphology support the conclusion that small crystallites in doped crystals are also present.

PHOTOELECTRICAL PROPERTIES OF CdMnSe THIN FILMS

The studied semiconductor structure for photovoltaic cells is composed of SnO2-coated glass and CdMnSe thin film. A study is made by examining the photoluminescence from the surface of the CdMnSe thin film with laser power and sample temperature for an as-grown, then an air-annealed thin film and undergone CdCl2 treatment. Thin films of Cd1-xMnxSe (x=0.02) were grown on a glass substrate. The lifetime of charge carriers under pulsed illumination was determined from the kinetic decay of the photocurrent. The study of relaxation curves of nonequilibrium photoconductivity under the influence of laser radiation confirmed the presence of two recombination channels - intrinsic and impurity. Photocurrent relaxation occurs through fast and slow recombination channels. The fast relaxation time τ = 6 μs associated with the intrinsic transition and the slow relaxation time is due to impurity excitation and τ = 22 μs. The photoluminescence spectra of thin films of Cd1-xMnxSe (x=0.02) were studied. In the photoluminescence study, two maxima are observed due to donor-acceptor recombination and intracenter transition of Mn atom.

Ca co-doped CsI(Tl) crystal scintillator for γ- and X-ray detecting applications

The vertical Bridgman method is used for the growth of single-doped CsI(Tl) and CsI(Tl:Ca) crystals, which has been evaluated for feasible applications in X-ray imaging and radiation detection. The powder XRD technique study reveals a single-crystalline phase of the grown crystal. The emission spectra show a broad emission band with a maximum of 540 nm under X-ray excitation, along with the characteristic emissions that arose from the Tl+ intra-center transition. The scintillation qualities of the grown crystal were studied via the pulse height spectra, scintillation light yield, energy resolution, and scintillation decay time. Pulse shape discrimination (PSD) was investigated under γ-rays and α-particles excitation sources. Moreover, the X-ray imaging application was investigated by an X-ray synchrotron at the Synchrotron Light Research Institute to study the performance of the grown crystal. These preliminary investigations suggest that the grown CsI(Tl:Ca) crystal can be one of the promising crystals for γ- and X-ray detecting applications.

Impact of Fe-impurity centers on phonon subsystem of wurtzite-type ZnO

The paper addresses the question of the structural and vibrational properties of Fe-doped ZnO in the wurtzite structure with a lattice without an inversion center. The ZnO crystals containing Fe impurities in different charged states are studied by means of density functional theory (DFT) and potential-based methods. A first-principles simulation is performed to determine the local atomic configurations near the considered defects. The DFT results are compared with those obtained using the shell model. Both approaches give a similar pattern of lattice distortion around the Fe-impurity centers occupying cation sites in the isoelectronic charge state Fe2＋ and single-positive-charge state Fe3＋. The phonon local symmetrized densities of states projected onto the displacement of ions surrounding the defects are calculated by a well-established recursion method. The frequencies of defect vibrations of various symmetry types induced by Fe impurities are determined. The calculations made it possible to interpret the structure of the phonon sideband accompanying the zero-phonon line in polarized emission spectra attributed to the Fe3＋ intracenter transitions, as well as to estimate the role of Fe ions in the formation of observed peaks.

Temperature‐Induced Transformation of the Atomic Configuration of the BO2* Defect in Boron‐Doped Czochralski Si

In this study, the new data concerning the electronic and vibrational properties of the BsO2i* defect in Czochralski‐grown boron‐doped silicon are reported. In silicon subjected to treatment at elevated temperatures, a new boron‐related defect is detected. An additional intracenter electronic transition for boron associated with the revealed defect is observed. The defect is identified as BsO2i due to the linear dependence of its formation efficiency on the boron content and the quadratic dependence on the oxygen concentration. The revealed complex is formed synchronously with the annealing of the previously identified BsO2i* defect. The detected complex is formed as a result of temperature transformation of the atomic configuration of the BsO2i* defect. The transformation occurs with activation energy of 2.59 eV. The local vibrational modes associated with both configurations of the BsO2i* complex are identified. The results of the study suggest that the BsO2i* defect in both configurations exists in a wide temperature interval, impacts the optical and electronic properties of the material, and must be taken into consideration when developing Si:B‐based devices.

2D Time-Domain Spectroscopy for Determination of Energy and Momentum Relaxation Rates of Hydrogen-Like Donor States in Germanium.

We present measurements of the coherence times of excited states of hydrogen-like arsenic impurities in germanium (Ge:As) using a table-top two-dimensional time-domain spectroscopy (2D-TDS) system. We show that this laboratory system is capable of resolving the coherence lifetimes of atomic-like excited levels of impurity centers in semiconductors, such as those used in solid-state quantum information technologies, on a subpicosecond time scale. By fitting the coherent nonlinear response of the system with the known intracenter transition frequencies, we are able to monitor coherent population transfer and decay of the transitions from the 2p0 and 2p± states for different low excitation pulse fields. Furthermore, by examining the off-diagonal resonances in the 2D frequency-domain map, we are able to identify coherences between excited electronic states that are not visible via conventional single-frequency pump-probe or Hahn-echo measurements.

Impurity-induced enhancement of parity-forbidden optical intracenter transitions of shallow donors in silicon

Different defects in silicon crystals cause lattice distortion that may violate selection rules for certain intracenter optical transitions of hydrogen-like donor centers. Perturbations due to large concentration of substitutional donors enhance all parity-forbidden atomic transitions, but cause large concentration broadening of the transition lines, prohibiting observation of transitions at close energies. Substitutional residual carbon atoms in silicon induce, in contrast, a weak-to-moderate broadening of donor absorption lines enabling the resolution of some parity-forbidden intracenter transitions in impurity absorption spectra at moderate donor densities. Binding energies of several series of s- and d-type states were obtained by resolving of intracenter transitions terminating in these states, by low-temperature infrared absorption spectroscopy in carbon-rich and/or heavily doped silicon crystals.

Valley-orbit splitting of lithium-related donors in silicon

The energy spectra of lithium-related donors in silicon are revisited by temperature-dependent infrared absorption spectroscopy. We determine the valley-orbit splitting of an interstitial isolated lithium donor and its complex with residual oxygen in float-zone grown crystals doped from the melt. Lithium-oxygen donors exhibit a temperature evolution of their absorption spectra similar to that of substitutional single-electron group-V donors, while its valley-orbit splitting is corrected upwards to +9.865(5) meV. For the lithium donor, the previously reported inverted valley-orbit splitting is not confirmed since the components of different symmetry are not spectrally resolved in our study. Instead, the chemical shift of the ground state of the donor, having a symmetry different from that of group-V donors, was corrected to a slightly larger value of 1.82(2) meV. Intracenter transitions from the split-off ground state of the Li donor exhibit an anomalous broadening. In addition, several Rydberg-like high excited states, up to $7{h}_{\ifmmode\pm\else\textpm\fi{}}$, of lithium-related centers were determined from absorption spectra of moderately doped samples.

Non-Radiative Transitions of Holes on Mercury Vacancies in Narrow-Gap HgCdTe

Mercury vacancies, acting as double acceptors, are the dominant point defects in ternary HgCdTe alloys. Though HgCdTe is one of the leading materials in infrared optoelectronics, the energy spectra of the vacancies are still a matter of some debate. This work investigated the rates at which holes are captured to a singly ionized mercury vacancy via acoustic phonon emission in narrow-gap Hg1−xCdxTe with technologically relevant x~0.22. Combined with the calculated rates of intracenter transitions, the data allow one to predict the most pronounced optical transitions in the emission spectrum of a double-charged acceptor. The results are sustained by the photoluminescence spectroscopy in the terahertz domain, allowing one to identify the emission band that is related to neutral vacancies.

New properties of boron-oxygen dimer defect in boron-doped Czochralski silicon

Silicon doped with boron is the most widely used material in modern microelectronic devices based on p-Si. Therefore, it is important to thoroughly understand boron’s role in the processes of defect-impurity interaction in Si both on growing the material and during operation of devices. In this work, interactions of boron with oxygen in Si are investigated by studying boron absorption intracenter transitions, which are known to be highly sensitive to the local environment. In boron-doped Si, two lines with maxima at 228 and 261.3 cm−1 were detected. The linear dependence of lines intensity on boron concentration and the quadratic on oxygen content testifies that the defect responsible for the lines can be identified as BsO2i. The observed absorption lines correspond to the transitions from the ground to the excited states of boron, which are shifted toward lower frequencies relative to the main transitions due to a deformation perturbation from neighboring oxygen atoms. The activation energy of annealing and ionization energy of defect are determined. The properties of the registered ВsO2i defect differ from the known ВsO2 associated with the light-induced degradation of solar cells by local configuration. The data obtained testify that the ВsO2i defects with different properties can be formed in Si and must be taken into account when developing Si:B-based devices because they can play an important role in charge carrier transfer and affect the electrical and optical parameters of the material.

Excitation of intracenter terahertz radiation by plasma oscillations in electron–hole liquid

A new mechanism for the excitation of impurity related terahertz radiation in semiconductors under the conditions of exciton condensation into an electron–hole liquid is reported. The interaction of impurity centers with plasmons localized on droplets of an electron–hole liquid produces ionization of the centers. The subsequent capture of nonequilibrium charge carriers by ionized impurities is followed by terahertz intracenter radiative transitions. In these processes, impurity centers play the role of antennas that convert the near electromagnetic field of plasmons on droplets of an electron–hole liquid into detected radiation. The main experiments were carried out on lithium-doped silicon crystals at helium temperatures under conditions of interband photoexcitation. A theoretical model of the excitation of impurity centers by localized plasmons is developed, which explains the main regularities observed in the experiment.

Electrical properties of α-Ga2O3 films grown by halide vapor phase epitaxy on sapphire with α-Cr2O3 buffers

We report on growth and electrical properties of α-Ga2O3 films prepared by halide vapor phase epitaxy (HVPE) at 500 °C on α-Cr2O3 buffers predeposited on sapphire by magnetron sputtering. The α-Cr2O3 buffers showed a wide microcathodoluminescence (MCL) peak near 350 nm corresponding to the α-Cr2O3 bandgap and a sharp MCL line near 700 nm due to the Cr+ intracenter transition. Ohmic contacts to Cr2O3 were made with both Ti/Au or Ni, producing linear current–voltage (I–V) characteristics over a wide temperature range with an activation energy of conductivity of ∼75 meV. The sign of thermoelectric power indicated p-type conductivity of the buffers. Sn-doped, 2-μm-thick α-Ga2O3 films prepared on this buffer by HVPE showed donor ionization energies of 0.2–0.25 eV, while undoped films were resistive with the Fermi level pinned at EC of 0.3 eV. The I–V and capacitance–voltage (C–V) characteristics of Ni Schottky diodes on Sn-doped samples using a Cr2O3 buffer indicated the presence of two face-to-face junctions, one between n-Ga2O3 and p-Cr2O3, the other due to the Ni Schottky diode with n-Ga2O3. The spectral dependence of the photocurrent measured on the structure showed the presence of three major deep traps with optical ionization thresholds near 1.3, 2, and 2.8 eV. Photoinduced current transient spectroscopy spectra of the structures were dominated by deep traps with an ionization energy of 0.95 eV. These experiments suggest another pathway to obtain p–n heterojunctions in the α-Ga2O3 system.

Structural, optical and photoelectric properties of Tb doped ZnO thin films for device applications

In this work we have investigated the effect of thermal annealing of thin ZnO:Tb3+ films obtained by the combined method of electrochemical deposition of Tb ions onto silicon substrates and subsequent reactive magnetron sputtering of a zinc target on the substrates on the structural properties, optical and photoelectric properties. ZnO:Tb3+ films with annealing temperature up to 700 °C are close-packed structures without porosity, with an almost smooth surface and dense packing of crystallites. The crystallite size and film thickness increase insignificantly with increasing annealing temperature. The films under investigation exhibit strong green luminescence with three bands in the spectral region of ∼1.9–2.6 eV, caused by intracenter 4f transitions on Tb3+ ions without changing the redistribution in the relative intensity of three bands with an increase in the annealing temperature. The study of the photoelectric properties showed that with an increase in the bias voltage, the maximum of the spectral sensitivity in absolute value increases and shifts to the short-wavelength region, while with an increase in the annealing temperature, the photosensitivity decreases, and upon annealing at 1100 °C it is practically absent.

PHOTOELECTRIC PROPERTIES OF MnIn2S4 SINGLE CRYSTALS

The spectral distribution of the photoconductivity and the temperature dependence of the photocurrent of MnIn2S4 single crystals are investigated. The intrinsic, impurity photoconductivity and a maximum at an energy of 2.69 eV, which is associated with the intracenter transition of Mn2+ ions (6A1→4A1), are revealed
 in the photoconductivity spectrum. The region of the wavelengths of 600–1000 nm appears with an excess of manganese in the crystals and is caused by a donor defect. At temperatures of 80—145 K, the increase in the photocurrent is associated with the thermal depletion of the adhesion levels. The activation energy of the adhesion levels is determined.

Intracenter dipole transitions of a hydrogen-like boron acceptor in diamond: Oscillator strengths and line broadening

Substitutional boron in diamond acts as an acceptor center with the ionization energy of about 372 meV. Unlike its analogues in elemental semiconductors (silicon, germanium), boron bound states are poorly described by the electronic mass approximation due to the large ionization energy and the small spin-orbit splitting of the valence band in diamond. Thereby experimental investigations appear to be a main way of study of discrete electronic states of boron in diamond.In this paper, we report infrared (IR) absorption spectroscopy results obtained for HPHT-grown single crystal diamonds doped with natural boron (80% 11B – 20% 10B), and with isotopically enriched boron (99% 11B – 1% 10B). A moderate dopant concentration (1016–1017 cm−3), low concentration of lattice defects and vanishing thermal impact at cryogenic temperatures provided significantly reduced line broadening so that more than 60 boron transitions could be spectrally resolved. Mathematical approximation of the spectral lines revealed the main empiric features of zero-phonon dipole intracenter transitions: their energy, linewidth and integrated absorption. This allowed us to calculate oscillator strength of the boron acceptor transitions in diamond. The Lorentzian shape of the spectral lines indicates dominating uniform broadening of the transitions. Together with the relatively broad linewidths (0.1–0.5 meV) it reveals strong electron-phonon interaction in boron-doped diamond.

The Features of Infrared Absorption of Boron‐Doped Silicon

Herein, the influence of heat treatment at 400 °C on the spectrum of boron intracenter transitions in silicon using IR absorption spectroscopy is investigated. In the transition region from the ground 1Γ8+ state associated with the p3/2 valence band of Si to the odd‐parity excited states of boron, a new absorption line with its maximum at 261.3 cm−1 is observed in the thermally treated boron‐doped Cz‐Si. Oxygen is a component of defect that is responsible for the detected absorption line. Perturbation of boron atoms due to the inhomogeneous stress effect from neighboring oxygen atoms results in a frequency shift in the main boron transition. The defect associated with 261.3 cm−1 line is also observed in as‐grown silicon. The defect disappears during annealing at 550 °C. The concentration of defects and binding energy of the 1Γ8+ ground state are estimated assuming that the line observed belongs to the transition of boron 1Γ8+ → 2Γ8−, subjected to the perturbation from a neighboring oxygen atom.

Activation ability of Gd dopant in the ZnSe single crystals

Rare-earth elements are widely used as doping materials as they considerably change the semiconductor optical, electrical, magnetic and radiative properties. This work explores the influence of Gadolinium (Gd) as a dopant on the radiative, optical and magnetic properties of the Zinc Selenide (ZnSe). ZnSe single crystals were grown by the physical transport method and doped during the growth process using a GdSe source. A wide range of characterization equipment was employed to analyze the obtained ZnSe:Gd single crystals. Intracenter radiative transition of the Gd has not been detected. Gd ions activate the background impurities, causing radiative transitions from the conduction band to the Cu2+ level and intracenter transitions within V3+, V2+ and Cr2+ ions. At the same time, the edge band intensity is dependent on the dopant concentration. Optical transmittance decreases, but the position of the fundamental absorption band is unchanged. Single crystals have a zinc blende crystal structure, and Gd ions do not form complexes with native defects or background impurities.

Enhanced exciton binding energy, Zeeman splitting and spin polarization in hybrid layered nanosheets comprised of (Cd, Mn)Se and nitrogen-doped graphene oxide: implication for semiconductor devices

The exciton properties of (Cd,Mn)Se-NrGO (nitrogen doped reduced graphene oxide) hybrid layered nanosheets have been studied in a magnetic field up to 10 T and compared to those of (Cd,Mn)Se nanosheets. The temperature dependent photoluminescence reveals the hybridization of inter-band exciton and intra-center Mn transition with enhancement of the binding energy of exciton-Mn hybridized state (80 meV with respect to 60 meV in (Cd,Mn)Se nanosheets) and increase of exciton—phonon coupling strength to 90 meV (with respect to 55 meV in (Cd,Mn)Se nanosheets). The circularly polarized magneto—photoluminescence at 2 K provides evidence for magnetic field induced exciton spin polarization and the realization of excitonic giant Zeeman splitting with g eff as high as 165.4 ± 10.3, much larger than in the case of (Cd,Mn)Se nanosheets (63.9 ± 6.6), promising for implementation in spin active semiconductor devices.

Photoluminescence of CdTe thin films doped with Yb

In this paper, for the first time, we investigated the influence of ytterbium doping on the radiative recombination processes in CdTe thin films deposited on a glass substrate by means of the close-spaced vacuum sublimation technique. This allowed to determine the nature and energy structure of the intrinsic defects, residual impurities and complex acceptor centers in the films. It was found that the optical spectra of such films reflect the processes of formation of free excitons and radiative recombination with the participation of bound excitons as well as donor-acceptor pairs. For the first time, emission associated with intracenter optical transitions of Yb3+ ions in CdTe was detected and its nature was established. CdTe: Yb thin films have been shown to have very high crystalline and optical quality. It is established to be a result of a strong decrease in the concentration of shallow acceptor centers associated with residual atoms and cadmium vacancies. This is due to the formation of complex centers with the participation of Yb3+ ions and the mentioned acceptors, i.e by gettering of residual atoms in CdTe:Yb thin films. In addition, in this case we can expect the conversion of their conductivity from p-to n-type. These results open a unique opportunity to control the crystalline and optical quality, as well as the conductivity type of semiconductor crystals II-VI by doping them with rare earth elements.

Intrinsic emission and electron-hole trapping centers in irradiated [formula omitted

The nature of intrinsic emission and the mechanisms of creation of electron-hole trapping centers in Na2SO4 in a wide temperature range of 15–300 K under irradiation with photons in the energy range of 4÷12.5 eV have been investigated by the methods of vacuum-ultraviolet and thermoactivation spectroscopy. Crystals of Na2SO4 were grown from a saturated aqueous solution by the method of slow evaporation. The results of XRD showed the crystalline structure of the prepared material. It is shown that intrinsic emission appears during the recombination of electrons from the s state of the conduction band with nonequivalently spaced holes SO4−. Electronic transitions from the orbital of the valence band to the conduction band have been identified. For the first time, the band gap was experimentally estimated within 4.9–5.1 eV, and the energy of intracenter electronic transitions at the created electron-hole trapping centers was also estimated.